In the paper-making industry, machines called "pulp machines" or "pulpers" prepare the paper pulp by breaking down cellulose materials which can be totally or partly materials to be recycled, such as old papers, boxes etc. In order to achieve this, the pulping machine is made of a fixed, open tub having a rotor in the form of a turbine. This tub is filled with water and with a certain amount of cellulose materials, and the rotor is then started up. The rotor causes energetic stirring of the water and the disintegration of the cellulose material which occurs in the form of dispersion or of a suspension and is discharged through perforations provided in the bottom of the tub.
In the standard kind of pulping machines, the concentration of dry cellulose materials has to remain quite weak, below 7% and generally about 5%, since otherwise the suspension is not liquid enough to be able to flow away and be discharged through the perforated sheet of metal forming the grating which usually surrounds the turbine.
In spite of this dilution, the discharge grating gradually becomes blocked up, as much by foreign bodies which are mixed with the cellulose material (clips, iron wires, string, etc) as by pieces of cellulose which have still not been broken down. Thus it is necessary to stop the pulping machine and to empty it, which involves a break in the production and a great loss of material, as well as being a source of pollution resulting from the disposal of these remnants.
In practice, the pulping machine is started up and it is interrupted when the flow of the discharged suspension falls below a certain value. The pulping machine is then half empty, which for a pulping machine of 30 m.sup.3 represents 15 m.sup.3 of the suspension with 5% of dry material, that is about 750 kg of dry material containing about 10% to 15% of impurities and 85% of good fibre. If the pulping machine is stopped and emptied three times in 24 hours, this means a loss of 1.3 tons of good fibre for 200 kg of impurities.
These figures explain the existing interest in trying to recover this fibre and avoid the pollution which results by its rejection.